The present invention relates generally to apparatus for depositing coatings on substrates and, more particularly, to apparatus for sputter coating nuclear fuel pellets.
Nuclear pellets have the shape of generally right circular cylinders. The fuel pellets contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The pellets are placed in end-to-end abutment within a cladding tube (typically made of a zirconium alloy or stainless steel) to make a fuel rod. The fuel rods usually are grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.
As is known to those skilled in the art, the life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. Burnable absorbers are materials (such as boron, gadolinium, samarium, europium, and the like) which have a high probability (or cross section) for absorbing neutrons, resulting in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons, all while producing no new or additional neutrons. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly. During reactor operation, the burnable absorbers are progressively reduced in amount, resulting in a longer life at relatively constant fission level for the fuel assembly. Longer fuel assembly life means less frequent nuclear reactor fuel assembly replacement. This refueling process is costly and time consuming.
U.S. Pat. No. 3,427,222 discloses fusion bonding a burnable poison (also known as a burnable absorber) layer to the surface of a nuclear fuel pellet substrate. Commonly assigned U.S. patent application Ser. No. 526,712 ("Coating a Nuclear Fuel with a Burnable Poison" by Kenneth C. Radford, filed Aug. 26, 1983) discloses sputtering as a method for depositing a layer of burnable poison on nuclear fuel pellets. The present invention discloses apparatus for accomplishing the sputtering operation in a unique manner.
Sputtering is a process for depositing a coating on a substrate, as is known to those skilled in the art. An inert gas is introduced into a vacuum chamber containing an anode, the substrate, and a target cathode. A high voltage is applied across the anode and target cathode. The gas molecules are ionized and strike the target cathode. This causes atoms and/or molecules of the target cathode to be sputtered from the target cathode's surface. The substrate is positioned to intercept this sputtered target material which forms an adherent coating upon striking the substrate. Sputtering is a generally line-of-sight coating process.
A sputtering apparatus is disclosed in U.S. Pat. No. 4,080,281 for making metal films, such as electrical resistors or electrodes of electrical capacitors. The substrates are placed in an annular cylindrical cage which serves as the anode. The outer coaxial, annular cylindrical target cathode circumferentially surrounds the cage to sputter radially inward on the cage from all directions, including downward. The inner coaxial, rod target cathode lies along the cage's longitudinal axis to sputter radially outward on the cage from all directions, including downward. The cage rotates to cause the metal films to be more uniform.
U.S. Pat. Nos. 3,562,140 and 3,632,494 disclose sputtering apparatus for coating razor blades with chromium. The razor blades are on a rotating drum. The razor blades are caused to rotate with respect to the drum as the drum rotates, to allow both edges of double-edge blades to be coated from a single target cathode.